1. Field of the Invention
The present invention relates to electrical connectors and more particularly high I/O density array connectors.
2. Brief Description of Prior Developments
The drive to reduce the size of electronic equipment, particularly personal portable devices, and to add additional functions to such equipment, has resulted in an ongoing drive for miniaturization of all components, especially electrical connectors. Efforts to miniaturize connectors have included reducing the pitch between terminals in single or double row in-line connectors, so that a relatively high number of I/O or other lines can be interconnected by connectors that fit within tightly circumscribed areas on the circuit substrates being electrically connected. The drive for miniaturization has also been accompanied by a shift in preference to surface mount techniques (SMT) for circuit board manufacture. The confluence of the increasing use of SMT and the required fine pitch of in-line connectors has resulted in approaching the limits SMT for high volume, low cost operations. Reducing the pitch of the terminals increases the risk of bridging adjacent solder pad or terminals during reflow of the solder paste. To satisfy the need for increased I/O density, array connectors have been proposed. Such connectors have a two dimensional array of terminals mounted on an insulative substrate and can provide improved density. However, these connectors present certain difficulties with respect to attachment to the circuit substrates by SMT techniques because the surface mount tails and most of the terminals must be beneath the connector body. As a result, the techniques used must be highly reliable because it is not possible to visually inspect the solder connections or repair them if faulty. In the mounting of an integrated circuit (IC) on a plastic or ceramic substrate the use of ball grid array (BGA) and other similar packages has become common. In a BGA package, spherical solder balls are positioned on electrical contact pads of a circuit substrate by means of a paste typically applied with a stencil or supporting device. The unit is then heated. The IC may thereby be connected to the substrate without need of external leads on the IC.
While the use of BGA and similar systems in connecting an IC to a substrate has many advantages, a corresponding means for mounting an electrical connector on a printed wiring board (PWB) or other substrate has yet to be developed. It is, for example, important that the completed solder balls be of a similar size so that in the final application the balls will reflow and solder evenly to a printed circuit board substrate. Any significant differences is solder ball size on a given substrate could cause problems when the final assembly is applied to a printed circuit board. The final size of the ball is dependent on the total volume of solder initially available including the solder paste and the solder balls. In applying balls to a connector contact, this limitation could be a particular problem since variations in the volume of the connector contact adds to the potential variability.
Another problem presented in soldering connectors to a substrate is that connectors often have insulative housings which have relatively complex shapes such as having numerous cavities. Such housings may, therefore, tend to become warped or twisted either initially or after heating to reflow the solder balls. Such warping or twisting of the housing can cause a dimensional mismatch between the final connector assembly and the printed circuit board, resulting in unreliable soldering because the balls are not sufficiently in contact with the solder paste on the PWB prior to soldering.
A need, therefore, exists for a means of using of BGA or a similar package to fix an electrical connector on a substrate.